Various processes for the preparation of compounds of formula (I) have been reported, for example Briggs et al, J. Chem. Soc. Perkin. Trans. I, 1985, 795 relates to a multistep synthesis of the 3,4-dihydroxyhexane-2,5-dione starting from the tartaric acid, or Bassignani et al, J. Org. Chem., 1978, 43, 4245 relates to the synthesis of the 3,4-dihydroxyhexane-2,5-dione by oxidizing the expensive 2,5-dimethylfuran with the toxic and expensive KClO3/OsO4 system.
Another reported method to synthesise compounds (I) is the reductive dimerisation of glyoxals. However, despite the fact that this reaction allows to obtain compound (I) as the final product in only one step and from easily available glyoxals (II), said reaction has attracted only little attention.
To our knowledge, the prior art discloses only four different processes to dimerise glyoxals:    a) European patent EP 368211 B1 discloses an electrochemical process to dimerise alkyl glyoxals with a yield of about 47%.    b) Büchi et al, J. Org. Chem., 1973, 38, 123 teach the use of metallic Zn to promote the dimerisation of methyl glyoxal with a yield of about 55%.    c) Clerici et al, J. Org. Chem., 1989, 54, 3872 teach the use of TiCl3 to promote the dimerisation of phenylglyoxal with a yield of about 30%.    d) Russell et al, J. Am. Chem. Soc., 1966, 88, 5498 teach the use of Cu(I), obtained in situ from Cu(NO3)2 and HOCH2SO2Na.2H2O, to promote the dimerisation of phenylglyoxal with a yield of about 60-70%.
The methods mentioned hereinabove suffer of their low yield (generally lower than 70%) and/or of the use of at least 0.1 equivalent of heavy metal salt which implies problems of purification of the final product and of waste treatment. Therefore there is still a need to find a process to achieve the reductive dimerisation of glyoxals with high yield and without using heavy metals as coreactants.